Tilted tubules

杨柳

MURATA/MACMILLAN
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Researchers have long wondered how plants carry out microtubule nucleation, as they lack microtubule organizing centers. Takashi Murata, Matsuyasu Hasebe, and colleagues (National Institute for Basic Biology, Okazaki, Japan) now show that -tubulin promotes microtubule nucleation off of existing microtubules in higher plants. Moreover, new microtubules branch off from the sides, rather than the ends, of existing microtubules.
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$ ~( g' E& S2 I: y7 I: I' TPlant cells form ordered microtubule arrays during interphase to enable cell shaping and orientation. To understand how these arrays form, Murata's group watched fluorescently labeled microtubules grow and shrink in live tobacco cells. They saw that all newly forming microtubules branched off either from existing tubules (at a 40° angle) or at a site where a microtubule had existed but then depolymerized only seconds earlier.2 U: F) m& a2 a- D2 R# b1 k
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Using in vitro analysis, the team confirmed that already-established microtubules are required for new microtubule nucleation. They also found that -tubulin, a common component of microtubule organizing centers in other organisms, is essential for nucleation and localizes at microtubule branch points.
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9 @# X- A3 {4 Y. F3 m/ yMurata is currently trying to identify accessory proteins that may facilitate -tubulin movement from the cytoplasm to the sides of microtubules, and exploring links to other systems. "Although green plants are the only organisms that exhibit this branching type of microtubule nucleation so far, the branching pathway of microtubule nucleation may be found in other eukaryotic cells since cytoplasmic -tubulin is found even in animal cells," he says.! P3 B+ o% E+ v1 s& J
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Reference:
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Murata, T., et al. 2005. Nat. Cell Biol. 10.1038/ncb1306.(New plant microtubules branch off from e)